A hero Marine from Brooklyn who lost his leg from a roadside bomb in Iraq amazingly survived an 800-foot fall down a New Hampshire mountain when he and two ice-climbing pals were swept away in an avalanche.

Zeier, who was climbing with a custom-made prosthesis, was among an expedition of 12 climbers tackling the difficult Mount Washington to raise funds and awareness for the Special Operations Warrior Foundation.

At about 4:30 p.m. Thursday, Zeier headed up a gully with a 40-degree incline, roped together with the Politzes as they and the other nine climbers headed for the summit to spend the night in a weather station.

Then, abruptly, a large slab of snow above them gave way and slammed into the climbers — sending nine of them hurtling down the icy gully.

“I fell about 200 feet,” said Chris Lanka, one of six to plunge that distance. “It was terrifying.”

But Zeier and the Politzes kept going, flying 600 more feet down the gully in the massive rush of snow.

Andy Politz “fought like a tiger” to plunge his axe into the ice to halt their free fall, but was unable to do so, said climber Scott Blanchard.

When they finally stopped, Politz looked around — but didn’t see his son or Zeier.

“The moment I didn’t know where they were and thought they were submerged was terrifying,” Politz said.

“I got up, but I felt the bone grinding around, and then crawled,” he said. “I called out to them and realized they were just a few feet away.”

He suffered a compound fracture to his leg and later underwent surgery.

“We crawled for hours” toward a trail below, Politz said — as the others above rappelled down to look for them.

Zeier suffered a jammed hip and twisted shoulder from the dramatic plunge. His left leg had been amputated in 2010 because of the aftereffects of a 2006 explosion in Fallujah, Iraq, which had blasted him from a Humvee and left him badly injured.

“We’re fortunate everyone is safe, down the mountain in one piece,” Blanchard said yesterday — “especially the three guys who took the long fall.”